In a network of this type, the two storage elements have different nominal direct service voltages, and a reversible converter, which is better known to persons skilled in the art as a DC/DC1 converter, arranged between the two elements, is necessary in order to carry out the transfers of energy from one to the other.
A known problem consists of the initial charging of the storage element which has the higher nominal voltage, by the other element, up to the nominal voltage of the latter.
The technical problem is particularly difficult to solve when the objective is to charge an ultracapacitor with several tens or even hundreds of Farads, from a 12V battery for example, since a very substantial initial current requirement may arise.
In general, this current can not be supplied by the DC/DC converter itself. An additional device is therefore used in order to carry out the initial charging.
The simplest known device consists of a resistor which is interposed between the two storage elements.
This simple solution has several disadvantages:                if the resistor has a low ohmic value, in order to reduce the duration of the initial charging, then the charging current is substantial, which leads to use of a high-power, high-cost resistor;        if a resistor is selected which has an acceptable value in terms of power dissipated, for example 1 ohm, then the duration of the charging becomes prohibitive (over 15 minutes);        a resistor which is connected permanently between the elements leads to a substantial leakage current, and to discharge from one element to the other.        
In patent application FR2838572, the company VALEO discloses a solution which eliminates the aforementioned disadvantages.
During the initial charging of the first element with the higher nominal voltage, this solution consists of putting what is known to persons skilled in the art as a stepdown voltage converter into service between the output of the second element with the lower nominal voltage and the input of the DC/DC converter, until the moment when the voltage at the terminals of the first element is equal to the nominal voltage of the second.
The stepdown voltage converter consists substantially of a static switch (better known to the person skilled in the art as “buck converter”), for example of a transistor of the “MOSFET” type, the duration of closure of which, relative to the duration of opening, makes it possible to control the mean charging current.
Since the transistor functions by switching, the disadvantage of the high level of thermal dissipation of the resistor is eliminated.
However, this solution also has the disadvantage of reducing the overall output of the energy transfer between the storage elements.
In fact, the stepdown voltage converter is used only in the initial stage of charging of the first element from the second element, when the voltage at the terminals of the first element is lower than the nominal voltage of the second element.
In the case of application for a motor vehicle, in which the first element is an ultracapacitor and the second element is a battery, this situation is infrequent.
Most of the time, the voltage at the terminals of the ultracapacitor is greater than that of the battery, and the static switch of the stepdown voltage converter constitutes an additional resistive charge in series which detracts from the performance of the system, even if this switch is permanently closed.
In addition, the filter which is placed upstream from the stepdown voltage converter, and is designed to filter the currents which are switched both by this stepdown voltage converter and by the DC/DC converter, must filter the cut-off caused by this converter of the stepdown voltage type, meaning that this filter must be oversized.